Method of producing a pigment containing, cationic, high solids aqueous dispersion, aqueous dispersion containing pigments, and use thereof

ABSTRACT

A method of producing an aqueous dispersion of a pigment, and the aqueous dispersion of the pigment and the uses of the aqueous dispersion of the pigment. In the present method, an inorganic pigment is dispersed in water, in the presence of a dispersing agent. According to the present invention, the inorganic pigment comprises cationic calcium carbonate, especially precipitated calcium carbonate, which is dispersed using a cationic biopolymer, such as starch, in order to prepare an aqueous dispersion, the solids content of which is approximately 30-75% by weight. The dispersion generated can be used, among others, as pigment or filler for ink-jet paper.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates by reference subject matter disclosed in International Patent Application No. PCT/FI2014/050378 filed on May 19, 2014 and Finnish Patent Application No. 20135532 filed May 17, 2013.

TECHNICAL FIELD

The present invention relates to production of cationic pigments, especially aqueous dispersions which have a high dry matter content. In particular, the present invention relates to the method for example for the production of an aqueous dispersion which is used in the production of paper or cardboard.

The present invention also relates to the aqueous dispersion that comprises pigments, and the use of the aqueous dispersion.

BACKGROUND ART

The pigments which are used as paper and cardboard fillers are added, in the form of an aqueous dispersion, into the pulp slush in the headbox of the machine. Good retention of the pigments (adhesion to pulp fibres) usually requires the use of retention agents. If the pigment used is precipitated calcium carbonate, PCC, which is prepared in the vicinity of the paper or the cardboard mill, and which is generated, in the form of a slurry that comprises 20-25% of solid matter, during the process of the production of pigment, a separate retention agent is not necessarily needed, because the PCC particle has a cationic surface charge, in which case it adheres well to the anionic fibre network.

Transportation of a PCC slurry which is dilute, i.e. which comprises approximately 20-25% of solid matter, over long distances is out of the question for cost reasons. If the PCC is going to be used far from its place of production, the PCC slurry is typically concentrated for example by centrifuging it to a higher percentage of solid matter, after which the slurry is dispersed with an anionic dispersing agent, and the resulting aqueous dispersion that has a high dry matter content is transported to the place of application. Alternatively, it is possible to dry the dispersion to a dry anionic pigment. When the dispersion or the aqueous slurry that is made from the dry anionic pigment is added into the pulp slush, a cationic retention agent must be added, too.

Also, when ground calcium carbonate, GCC, is used, it is usually delivered to the application in the form of a dispersion having a high solid matter content, and in which case an anionic agent has been used as the dispersing agent. Alternatively, the GCC is delivered to the application in the form of an anionic pigment which is stabilised by means of a dispersing agent. Thus, a cationic retention agent is also needed when GCC is used.

Various dispersions and slurries of PCC are already known. Thus, the WO application publication 9730220 describes a method of producing paper which is filled with PCC, in which case a cationic polymer, for example starch, is mixed into the PCC slurry, before the mixture is added into the pulp slurry. After that, a retention agent is added into the pulp slurry. The resulting mixture is not stable.

EP application publication 0 790 135 describes a solution of producing paper which is suitable for ink-jet printing technology. A surface layer is applied onto the paper web, which layer comprises a cationic pigment. The surface layer is comprised of an aqueous suspension of the pigment and a binder. The percentage of the binder in the mixture of the aqueous dispersion of the pigment and the cationic binder is significantly high, and it is not possible to add the binder for example into the headbox of the paper machine, and thus use it as paper filler. This aqueous dispersion is not stable either.

US patent publication 8197641 describes a paper filler, the use of which requires that an aqueous slurry is formed of it, into which slurry another aqueous dispersion is added. After that, an aqueous slurry of starch is added, too. The resulting aqueous slurry is not stable. Moreover, use of the previously described mixture requires the addition of a retention agent.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a stable, aqueous dispersion which has a high dry matter content and which comprises pigments and which is to be used in the paper and cardboard industry, in which aqueous dispersion the surface of the pigment that is used as a filler is cationic, and the retention of which to the negatively charged pulp fibres is good and proceeds without a separate retention agent.

In particular, the purpose is to provide a dispersion, the stability of which is so good that pigment precipitation does not occur even during a storage period of several weeks, even though the viscosity of the dispersion is relatively low.

Another purpose is to provide a dispersion which, when used for surface-sizing of paper, gives the paper a good printability.

A further purpose is to provide a dispersion which, when used for coating of paper, gives the paper good optical properties, good colour densities and facilitates the rapid drying of printing ink.

The present invention is based on the idea that an aqueous dispersion which comprises pigments can be produced by using a method in which either a dry pigment or an aqueous slurry or paste of the pigment, having a pigment content of approximately 30-75% by weight, is dispersed by using a dispersing agent and possibly an additive, in order to generate a stable dispersion. In the present invention, the dispersing agent used is cationic, modified starch. The additive used is for example water.

The present invention provides an aqueous dispersion which has a high dry matter content, and which comprises pigments and a dispersing agent, which aqueous dispersion is characterised in that the dispersing agent is a cationic polymer, for example a native polymer or a polymer derived thereof, in particular a modified starch.

The new dispersion can be used as paper or cardboard filler, in which case, if needed, the dispersion is diluted with water and directed into the headbox of the paper or cardboard machine.

The dispersion can also be used for surface-sizing of paper or cardboard, in which case the dispersion is mixed into a cationic starch that is suitable for conventional surface-sizing, and the mixture is applied to the dried web.

A third application is the coating of paper or cardboard, in which case the dispersion is mixed into a coating paste, which is applied to the dried web.

More specifically, the method according to the present invention is a method of producing an aqueous dispersion of a pigment, wherein the inorganic pigment is dispersed into water in the presence of a dispersing agent, characterised in that the inorganic pigment comprises cationic calcium carbonate, which is dispersed using a cationic biopolymer in order to produce a stable dispersion which has a solids content of approximately 30-75% by weight.

The aqueous dispersion according to the present invention comprises an inorganic pigment and a dispersion agent, wherein the inorganic pigment comprises cationic calcium carbonate, that the dispersion agent comprises a cationic biopolymer and that the solids content of the aqueous dispersion is approximately 30-75% by weight, preferably 50-60% by weight, most preferably approximately 57% by weight.

The uses include a use of a dispersion as a paper or cardboard filler, in which case the dispersion is, if needed, diluted with water and directed into the headbox of the paper or cardboard machine; a use of a dispersion as a paper or cardboard filler, without the use of a separate retention agent; a use of a dispersion for surface-sizing of paper or cardboard, in which case the dispersion is mixed into a cationic starch that is suitable for conventional surface-sizing, and the mixture is applied to the dried web; a use of a dispersion for the coating of paper or cardboard, in which case the dispersion is mixed into a coating paste, which is then applied to the dried web; and a use of a dispersion as a pigment or filler for ink-jet paper.

Considerable advantages can be achieved with the present invention. Thus, as the results below demonstrate, by using cationic starch as a dispersing chemical, excellent stability and fluidity properties are achieved. The results also show that, when cationic pigments are used together with cationic surface starch, it is possible to improve the ink-jet printability, for example, regarding the sharpness of the printing quality. The opacity of the dispersed dry matter is excellent when dry.

The dispersion according to the present invention remains stable for a period of at least 2 weeks, preferably for at least 4 weeks. This provides a significant advantage, because the dispersion can be stored for a certain period of time, i.e. for several weeks. In this case, the dispersion can be prepared in a particular plant, from where it is then delivered to different end users, typically paper and cardboard mills. Thus, the end users do not have to invest in dispersing apparatus and high expertise in dispersing technology.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows schematically alternative ways of dispersing the pigments, according to the method of the present invention, and

FIG. 2 shows the ink-jet print quality on a surface-sized paper, with different surface-sizing agents.

DETAILED DESCRIPTION

As described above, according to the new technology, in the production of an aqueous dispersion that comprises pigments, either a dry pigment or an aqueous slurry or a paste of the pigment, the pigment content of which is approximately 30-75% by weight, is dispersed by using a dispersing agent and possibly together with an additive, in order to generate a stable aqueous dispersion having a high dry matter content. A cationic biopolymer is used as the dispersing agent.

The pigment content of the aqueous dispersion is preferably 50-60% by weight, especially approximately 57% by weight.

The dispersion generated by the present method remains stable for a period of at least 2 weeks, more preferably for at least 4 weeks, most preferably for more than 6 weeks.

In this patent application a stable dispersion means that at maximum approximately 15%, especially at maximum approximately 10%, more preferably at maximum 5%, in particular at maximum 1% of the solid matter of the dispersion precipitates during the observation period, in which case the observation period means a period of at least 2 weeks, preferably at least 4 weeks, most preferably more than 6 weeks.

The dispersion is preferably carried out in such a way that shear forces are directed at the mixture which comprises a dry pigment or an aqueous slurry or paste of the pigment; dispersing agent and possibly water.

The particle size of the dispersed pigment can be in the nano-range, typically 20-400 nm, more preferably more than 20 and less than 250 nm, especially 40-200 nm, in particular 40-100 nm. The pigment particle size can also be larger than this. Thus, the initial material used can be PCC products or structured pigments having a particle size in the micrometre range. Examples of these products are polynuclear, typically of regular shape, for example particle clusters and particle agglomerates, which comprise several interconnected, in particular spherical calcium carbonate particles. The size (maximum diameter) of the structures of polynuclear clusters or similar products, i.e. the agglomerates or clusters, may be even 6000 nanometres, typically at maximum 4000 nm and especially at maximum 2500 nm, most suitably approximately 1500 nm.

Based on the above, the particle size of the particles to be dispersed is thus generally 20-4000 nm, most preferably 20-2500 nm and especially 40-1500 nm.

Here, the particle size means the diameter of particles after they have been changed into spherical bodies.

Preferably, the pigment to be dispersed is in the form of a dry pigment, an aqueous slurry of the pigment or an aqueous paste of the pigment.

In one embodiment, a pigment is introduced into the dispersion, the charge level of which pigment is not rendered anionic at any stage during the time between the production and the dispersion of the pigment. It is thus possible to entirely avoid the use of separate anionic chemicals, such as polyacrylates.

However, in the prior art, a product generated in the production of calcium carbonate is treated to render it anionic before dispersion. The WO application publication 9730220 explicitly states that, before the PCC slurry can be mixed with the suspension, it is necessary to mix into the slurry an amount cationic polymer that is needed for the cationisation (page 9, line 23). In the case of the WO publication, the amount to be added must be significant, which indicates the anionic nature of the slurry.

In many applications, the pigment should have a good dry opacity.

In one embodiment that fulfils the above requirement of dry opacity, the surface area of the calcium carbonate pigment is approximately 5-100 m²/g, especially approximately 10-50 m²/g.

Examples of porous calcium carbonate materials, which are suitable for this application as well as for other applications, include those which are comprised of calcium carbonate material, the particles of which are composed of calcium carbonate agglomerates or clusters. These are described for example in EP patent 0 944 551, according to which the precipitated calcium carbonate crystals (PCC crystals) are in the form of polynuclear, typically regular, usually spherical clusters, which comprise several interconnected, in particular

spherical calcium carbonate particles having a particle size of approximately 40-400 nm, typically approximately 40-200 nm.

The clusters described in that particular patent have numerous internal reflection surfaces which improve the opacity of the pigment and the filler.

The pigment can be used as such but it is of course possible to use structured pigments, i.e. calcium carbonate, together with light-scattering pigments. In this way, it is possible to increase the diffuse reflections of light, in which case the probability increases that light beams hit a reflecting surface.

EP patent 0 944 551 also describes a method of producing porous pigments, i.e. in that case calcium carbonate clusters. The publication is herewith incorporated by reference.

The cationic biopolymer which is used in the production is preferably a cationic, modified, substituted starch or cationic sorbitol or maltodextrin.

A specific example worth mentioning is a suitable, modified, substituted starch that comprises cationic, nitrogen-containing substituents, such as amine or amide groups.

The nitrogen content of the modified starch is for example approximately 0.1-2% of its dry weight.

In a more preferable embodiment, the degree of substitution of the modified starch is 0.1-0.2, especially approximately 0.16.

The technology can be applied to various pigments, especially to mineral pigments. Examples are calcium carbonate, such as precipitated calcium carbonate (PCC) or ground calcium carbonate (GCC); talc; gypsum; titanium dioxide; and kaolin.

More preferably, the present solution is applied to PCC, especially to a PCC that has been dried after the production process of the PCC.

According another preferred embodiment, the present solution is applied to an aqueous slurry or an aqueous paste of PCC, which is obtained by filtration of the dilute PCC aqueous slurry that comes from the production process of the PCC.

In a third embodiment, the pigment is dry PCC or GCC or an aqueous slurry or an aqueous paste of PCC or GCC, having a pigment content of 30-60% by weight, and the dry pigment or pigment slurry or paste is stabilised by using an anionic dispersing agent.

In one embodiment,

-   -   the pigment content of the aqueous dispersion is approximately         30-75% by weight, preferably 50-60% by weight, most preferably         approximately 57% by weight, and     -   the dispersing agent content of the aqueous dispersion is         0.2-5.0% by weight, preferably 0.5-3.0% by weight, most         preferably 0.7-1.5% by weight, in particular approximately 0.7%         by weight.

One possibility is to direct the dry pigment or the aqueous slurry or aqueous paste of the pigment, the dispersing agent and, if needed, water, into the dispersing unit, which is typically a disintegration device operating according to the impact-repulsion mill principle.

The resulting dispersion can be used as a paper or cardboard filler, in which case, if needed, the dispersion is diluted with water and directed into the headbox of the paper or cardboard machine.

The dispersion can also be used as a paper or cardboard filler without the use of a separate retention agent.

In one embodiment the dispersion is used for surface-sizing of paper or cardboard, in which case the dispersion is mixed into a conventional cationic starch that is suitable for surface-sizing, and the mixture is applied to the dried web.

The dispersion can also be mixed into the coating paste, which is applied to the dried web.

The dispersion of the pigment can be carried out in various ways, for example according to the process schematised in FIG. 1.

As described above, the aqueous dispersion is prepared without the addition of a polyelectrolyte, such as polyacrylate, before, during or after the dispersion. Thus, in one embodiment, a dispersion is prepared, the aqueous phase of which is essentially polyelectrolyte-free. For example, the polyacrylate percentage is lower than 0.1%, most suitably lower than 0.01%, especially lower than 0.001%, of the dry matter content of the dispersion.

In a preferred embodiment, the aqueous dispersion obtained comprises a cationic starch derivative approximately 0.1-2.0% by weight, especially approximately 0.5-0.99% by weight, of the dry weight of the dispersion.

The new technology allows the production of for example an aqueous dispersion, the Z-potential of which is at least +1 mV, at a pH value of 9.5 and a dry matter content of 20%.

According to one embodiment, in the method of producing the pigment slurry, a inorganic pigment is dispersed into water in the presence of a dispersing agent, in which case the inorganic pigment comprises cationic calcium carbonate, which is dispersed by using a cationic biopolymer in order to generate an aqueous slurry, the solids content of which is approximately 30-75% by weight, and in which case the cationic calcium carbonate is a dry cationic precipitated calcium carbonate, which is obtained by drying the slurry coming from the production of the precipitated calcium carbonate. It is possible to add water into the dry PCC, before dispersion.

Because the solids content of the dispersion according to the present invention is considerably high, also somewhat larger, micro-sized pigment particles may be present in the dispersion, along with the nano-sized pigment particles, without impairing much the stability of the dispersion.

More preferred embodiments are examined with reference to the drawings.

Production of a dry cationic pigment and production of a dispersion of a cationic pigment as a two-stage process

The slurry coming from the production of pigment (PCC), the solids content of which slurry is 24%, is fed along the pipe 11 into the centrifuge 1, where some of the water is removed. The paste-like PCC slurry having a solids content of approximately 50% is removed from the centrifuge. The paste is fed along the pipe 12 to the dryer 4, and the product 5 which exits from the dryer along the pipe 13 is nearly 100% a dry cationic pigment (PCC), which can be delivered as such for use. In the place of use or at the producer of the pigment, it is possible to slurry the dry cationic pigment in a desired amount of water, to feed a dispersing agent into it and to disperse it. The resulting product is a cationic PCC dispersion having a typical solids content of approximately 50%.

Dispersing of an anionic pigment in water together with a cationic, modified starch, in order to form a cationic pigment dispersion

PCC slurry having a solids content of 24% is fed into the centrifuge 1, where some of the water is removed. The PCC slurry which is removed from the centrifuge along the pipe 16 typically comprises approximately 50% of solids and it is directed to the disperser 3, to which is added an anionic dispersing agent. The dispersed slurry is directed by line 17 to the dryer 4, where it is dried. The resulting product, a dried anionic pigment (PCC), is a commercial product which is known per se.

A dry anionic pigment produced in a way which is known per se can be modified, for example, either in the place of use of the pigment or at the producer of the pigment, to a cationic aqueous dispersion of pigment having a high dry matter content. The dry anionic pigment is slurried in a desired amount of water, a dispersing agent is fed into it and then it is dispersed. The resulting product is a cationic dispersion of PCC, having a typical solids content of approximately 50%.

Although the anionic PCC pigment itself is useful as paper filler, it is advantageous to modify the pigment to be cationic as described above, because the retention of the cationic pigment is good and does not necessarily require the use of a retention agent.

Ground calcium carbonate (GCC) is also delivered in the form of a dry anionic pigment or a dispersion of an anionic pigment. These products can also be rendered cationic in the way described above.

The following non-restrictive examples illustrate the new technology.

Example of Slurry Production

A PCC slurry having a low dry matter content was prepared, which slurry was filtered to form a paste that has a high dry matter content. This was dispersed by using a cationic modified starch (Vector 20157, Roquette) as the dispersion agent. The viscosity of the prepared cationic aqueous dispersion of the PCC having a high dry matter content, was 430 cP. The product was stored in an IBC container for a period of 4 weeks. After storage, it was found that the aqueous dispersion of the PCC having a high dry matter content had not settled, and the viscosity of the slurry was measured and found to be 100 cP.

Example 1 The Effect of the Pigment Content and the Dispersing Agent Content on the Viscosity and the Stability of the Dispersion

The pigment used in the experiment was dry PCC. The pigment was dispersed in water using a cationic modified starch (Vector 20157; Roquette) as the dispersing agent. The viscosity was determined by means of a Brookfield rotational viscometer, at 100 rpm (Br100) and 50 rpm (Br50). The unit of measurement of viscosity is mPa s. The precipitation tendency of the dispersion was measured by pouring the dispersion into a 100 ml calibrated beaker that has a diameter of approximately 2.0 cm, and then observing how the height of the bright water column (mm) increases over time, for a period of 2, 7 and 14 days.

The results are shown in Table 1. The table shows that the dispersion, which has a pigment content of 57% and a dispersing agent content of 0.7%, has a good stability (low settling, and no observed change as a function of time) and a viscosity within a suitable range.

TABLE 1 7 Dispers. Viscosity Viscosity 2 days days 14 days agent, % Pigment % Br100 mPas Br50 mPas pH mm mm mm 0.5 50.19 336 528 8.76 4 4 4 0.5 54.2 1364 2220 9 1 1 1 0.5 58.2 6700 9860 8.5 0.5 0.5 0.5 0.7 50.07 100 118 8.57 1 5 8 0.7 53.9 207 246 8.5 1 4 4 0.7 57.9 172 226 8.52 0.5 0.5 0.5 0.9 49.9 225 246 8.78 1 2 4 0.9 54.1 177 210 8.85 0.5 0.5 2 0.9 58.1 892 1624 8.57 0.5 0.5 0.5 1.1 50.1 155 180 8.6 1 1 1.5 1.1 53.9 297 348 8.6 0.5 0.5 1 1.1 57.9 1590 2360 8.75 0-0.5 0-0.5 0.5 1.3 50 259 308 8.5 0-0.5 1 1 1.3 53.9 396 466 8.62 0-0.5 0.5 0.5 1.3 58.1 3280 5840 8.59 3 3 4

Table 2 shows the viscosity of the dispersion as a function of time. The pigment was dispersed in water using a cationic modified starch (Vector 20157; Roquette) as the dispersing agent. The viscosity was determined by means of a Brookfield rotational viscometer, as above. The dispersion that has a pigment content of 57.5% and a dispersing agent content of 0.7%, has a suitable viscosity, which does not change much as a function of time.

TABLE 2 Time, h Viscosity, Br100, mPas Viscosity, Br50, mPas 0 170 240 1 350 412 2 404 428 3 370 396 4 398 420 5 380 396 6 500 700 11 320 470 23 336 504 30 504 700 48 404 462

Tables 1 and 2 show that the modified starch used renders the dispersion a good stability and good flow properties.

Example 2 Ink-Jet Printability of Surface-Sized Paper

The cationic PCC pigment dispersion, which was prepared from a semi-dry filter cake, by using a modified starch (Vector 20157) as the dispersing agent, was mixed into a conventional cationic starch that is used for surface-sizing. Three tests were carried out, namely 1) a control, where the surface had been treated exclusively with cationic surface starch; 2) anionic pigment was added into the surface starch; 3) cationic pigment was added into the cationic surface starch. The mixtures were applied onto the surface of paper, and ink-jet printing was carried out.

The imprints of the experiments are shown in FIG. 2. It can be seen that the sharpest print quality was obtained in test no. 3, in which cationic pigment was used. This shows that the use of cationic pigment in the surface-sizing agent of paper improves the ink-jet printability of paper, because it prevents the printing ink from spreading and being mixed into other printing inks.

Example 3 Ink-Jet Printability of Coated Paper

The cationic PCC pigment dispersion, which was prepared from a semi-dry filter cake, by using modified starch (Vector 20157) as the dispersing agent, was mixed into a coating colour. The results were compared to the commonly used titanium dioxide pigment. Three tests were carried out. A coating paste comprising 15% of titanium dioxide was used as a control (Test 1). In Test 2, half of the titanium dioxide, and in Test 3 all of the titanium dioxide was replaced with a cationic PCC pigment that is described above. The papers were coated, an ink-jet printing was carried out and the coated papers and the imprints were tested. Table 3 shows the results.

TABLE 3 Test 1, Test 2 Test 3 control 7.5% TiO₂ 0% TiO₂ 15% TiO₂ 7.5% cat. 15% cat. 0% cat. PCC PCC PCC Optical Brightness, % 89.2 90.1 92.4 properties Fluorescence 2.6 3.6 6.0 Opacity, % 96.7 96.6 96.8 — — — — — Inkjet test Colour density 1.08 1.14 1.23 results Blue Colour density 0.93 0.95 1.06 Magenta Colour density 0.79 0.83 0.92 Yellow Colour density 2.92 2.92 2.96 Black Drying rate 6.2 5.0 3.8 (60 s); ΔR20

The improved fluorescence (i.e., the difference in brightness as measured under UV light conditions and under normal light) is due to the reduced titanium oxide content of the coating colour. The optical density of the coating colours blue, magenta, and yellow were significantly increased by the cationic PCC. The drying of the printing ink was accelerated by the cationic PCC. The decrease in brightness of white paper, which was pressed against the printed surface, was lower than in the control test.

The present invention is not intended to be limited to the above embodiments, which are only examples. The present invention is intended to be widely applied within the scope of protection of the claims below.

The dispersion according to the present invention is a kind of an initial product, which can be used for many different purposes in the end-user facility, such as paper or cardboard filler, a component of the surface-sizing agent for paper or cardboard, or a component of the coating paste of paper or cardboard. This fact is particularly significant for the material logistics of paper or cardboard mills, because only one acquired initial product is needed to produce a variety of end products.

Because the dispersion remains stable and the particles of the dispersion remain cationic, it is possible to use the dispersion directly as a filler by feeding it (possibly diluted in water), without retention agents, into the headbox of the machine. The dispersion is also, for example when mixed into cationic starch, very well suited for surface-sizing of paper. Use of the pigment of the dispersion in the surface-sizing agent of paper improves the printability of the paper, particularly the ink-jet printability, because it prevents the printing ink from spreading and mixing into other printing inks.

When used in the paper coating, the pigment of the dispersion provides better optical results (i.e. better brightness, fluorescence and opacity) than provided by titanium dioxide. The print quality of the ink-jet printing printed in this way on coated paper is also better than the printing quality when using titanium dioxide, and printing ink dries faster.

While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A method of producing an aqueous dispersion of a pigment, wherein the inorganic pigment is dispersed into water in the presence of a dispersing agent, characterised in that the inorganic pigment comprises cationic calcium carbonate, which is dispersed using a cationic biopolymer in order to produce a stable dispersion which has a solids content of approximately 30-75% by weight.
 2. The method according to claim 1, wherein the dispersion obtained remains stable for a period of at least 2 weeks, more preferably for at least 4 weeks, most preferably for more than 6 weeks.
 3. The method according to claim 1, wherein dispersing the mixture, which comprises a dry pigment or an aqueous slurry or aqueous paste of the pigment, a dispersing agent and possibly water, by directing shear forces at the mixture.
 4. The method according to claim 1, wherein the solids content of the dispersion is 50-60% by weight, preferably approximately 57% by weight.
 5. The method according to claim 1, wherein the pigment to be dispersed is in the form of a dry pigment, an aqueous slurry of the pigment or an aqueous paste of the pigment.
 6. The method according to claim 1, wherein introducing into the dispersion of a pigment, charge state of which is not rendered anionic during the time between the production and the dispersion of the pigment.
 7. The method according to claim 1, wherein the surface area of the calcium carbonate pigment is approximately 5-100 m²/g, in particular approximately 10-50 m²/g.
 8. The method according to claim 1, wherein the pigment comprises calcium carbonate and another, in particular a light-scattering pigment, such as titanium dioxide, kaolin, gypsum, silicate-based material, talc or a different calcium carbonate.
 9. The method according to claim 1, wherein the pigment consists of calcium carbonate.
 10. The method according to claim 1, wherein the pigment consists of a porous calcium carbonate material, in particular it consists of a calcium carbonate material, the particles of which are formed of calcium carbonate agglomerates.
 11. The method according to claim 1, wherein the pigment comprises or consists of precipitated calcium carbonate.
 12. The method according to claim 1, wherein the cationic biopolymer is a cationic, modified, substituted starch or a cationic sorbitol or maltodextrin.
 13. The method according to claim 1, wherein the modified, substituted starch comprises cationic, nitrogenous substituents, such as amine or amide groups.
 14. The method according to claim 1, wherein the nitrogen content of the modified starch is approximately 0.1-2% of its dry weight.
 15. The method according to claim 1, wherein the degree of substitution of the modified starch is 0.1-0.2, preferably approximately 0.16.
 16. The method according to claim 1, wherein the pigment is precipitated calcium carbonate that has been dried after the process of the production of the precipitated calcium carbonate.
 17. The method according to claim 1, wherein the pigment is an aqueous slurry or aqueous paste of calcium carbonate, obtained by filtering the dilute aqueous slurry of precipitated calcium carbonate, which slurry is generated during process of the production of pigment, is precipitated calcium carbonate.
 18. The method according to claim 1, wherein the aqueous dispersion is prepared without adding a polyelectrolyte, such as polyacrylate, before dispersion, during dispersion or after it.
 19. The method according to claim 1, wherein a dispersion is prepared, the aqueous phase of which is essentially polyacrylate-free, especially its polyacrylate content is lower than 0.1%, most suitably lower than 0.01%, especially lower than 0.001%, of the dry matter content of the dispersion.
 20. The method according to claim 1, wherein the aqueous dispersion comprises a cationic starch derivative approximately 0.1-2.0% by weight, especially approximately 0.5-0.99% by weight, of the dry weight of the dispersion.
 21. The method according to claim 1, wherein the pigment content of the aqueous dispersion is approximately 30-75% by weight, preferably 50-60% by weight, most preferably approximately 57% by weight, and the dispersing agent content of the aqueous dispersion is 0.2-5.0% by weight, preferably 0.5-3.0% by weight, most preferably 0.7-1.5% by weight, in particular approximately 0.7% by weight.
 22. The method according to claim 1, wherein the dry pigment or the aqueous slurry or aqueous paste of the pigment, the dispersion agent and, if needed, water, is directed into the dispersion unit, which is typically a disintegration device operating according to the impact-repulsion mill principle.
 23. The method according to claim 1, wherein an aqueous dispersion is prepared, the Z-potential of which is at least +1 mV, at a pH value of 9.5 and a dry matter content of 20%.
 24. An aqueous dispersion which comprises an inorganic pigment and a dispersion agent, wherein the inorganic pigment comprises cationic calcium carbonate, that the dispersion agent comprises a cationic biopolymer and that the solids content of the aqueous dispersion is approximately 30-75% by weight, preferably 50-60% by weight, most preferably approximately 57% by weight.
 25. The aqueous dispersion according to claim 24, wherein the dispersion is stable, especially the dispersion remains stable for a period of at least 2 weeks, preferably a period of at least 4 weeks, most preferably a period of over 6 weeks.
 26. The aqueous dispersion according to claim 24, wherein the mixture which comprises a dry pigment or an aqueous slurry or an aqueous paste of the pigment, a dispersing agent and possibly water, is dispersed by directing shear forces at the mixture.
 27. The aqueous dispersion according to claim 24, wherein the particle size of the dispersed particle is in the nano-range, typically 20-400 nm, more preferably over 20 and less than 250 nm, especially 40-200 nm, in particular 40-100 nm.
 28. The aqueous dispersion according to claim 24, wherein the aqueous phase of the dispersion is essentially polyacrylate-free, especially its polyacrylate content is lower than 0.1%, most suitably lower than 0.01%, preferably lower than 0.001%, of the dry matter content of the dispersion.
 29. The aqueous dispersion according to claim 24, wherein the Z-potential of the dispersion is at least +1 mV, at a pH value of 9.5 and a dry matter content of 20%.
 30. The aqueous dispersion according to claim 24, wherein the surface area of the calcium carbonate pigment is approximately 5-100 m²/g, especially approximately 10-50 m²/g.
 31. The aqueous dispersion according to claim 24, wherein the pigment comprises calcium carbonate and another, especially a light-scattering pigment, such as titanium dioxide, kaolin, gypsum, silicate-based material, talc or a different calcium carbonate.
 32. The aqueous dispersion according to claim 24, wherein the pigment consists of calcium carbonate.
 33. The aqueous dispersion according to claim 24, wherein the pigment consists of porous calcium carbonate material, especially it consists of a calcium carbonate material, the particles of which are formed of calcium carbonate agglomerates.
 34. The aqueous dispersion according to claim 24, wherein the pigment comprises or consists of precipitated calcium carbonate.
 35. The aqueous dispersion according to claim 24, wherein the modified, substituted starch comprises cationic, nitrogen-containing substituents such as amine or amide groups.
 36. The aqueous dispersion according to claim 24, wherein the nitrogen content of the modified starch is approximately 0.1-2% of its dry weight.
 37. The aqueous dispersion according to claim 24, wherein the degree of substitution of the modified starch is 0.1-0.2, preferably approximately 0.16.
 38. The aqueous dispersion according to claim 24, wherein the pigment is precipitated calcium carbonate, which has been dried after the process of the production of the precipitated calcium carbonate.
 39. A dispersion according to claim 24, wherein the pigment content of the aqueous dispersion is approximately 30-60% by weight, preferably 50-60% by weight, most preferably approximately 57% by weight, and the dispersing agent content of the aqueous dispersion is 0.2-5.0% by weight, preferably 0.5-3.0% by weight, most preferably 0.7-1.5% by weight, in particular approximately 0.7% by weight, calculated from the dry weight.
 40. A use of a dispersion according to claim 24 as a paper or cardboard filler, in which case the dispersion is, if needed, diluted with water and directed into the headbox of the paper or cardboard machine.
 41. A use of a dispersion according to claim 24 as a paper or cardboard filler, without the use of a separate retention agent.
 42. A use of a dispersion according to claim 24 for surface-sizing of paper or cardboard, in which case the dispersion is mixed into a cationic starch that is suitable for conventional surface-sizing, and the mixture is applied to the dried web.
 43. A use of a dispersion according to claim 24 for the coating of paper or cardboard, in which case the dispersion is mixed into a coating paste, which is then applied to the dried web.
 44. A use of a dispersion according to claim 24 as a pigment or filler for ink-jet paper. 